The present invention relates to an infinitely variable transmission for automotive vehicles, and particularly to a torque split infinitely variable transmission especially suitable for front-wheel-drive vehicles.
To meet demands for increased shift comfort, improved driveability, and reduced fuel consumption and exhaust emissions, there have been developed various continuously variable transmissions (often abbreviated to xe2x80x9cCVTsxe2x80x9d) such as belt-drive CVTs and toroidal CVTs, in which a transmission ratio is steplessly variable within limits. In order to further enlarge a shifting range (a speed-change area), in other words, to widen the difference between the lowest transmission ratio and the highest transmission ratio of the CVT, in recent years, there have been proposed and developed infinitely variable transmissions (often abbreviated to xe2x80x9cIVTsxe2x80x9d) capable of controlling a transmission ratio to infinity. One such infinitely variable transmission has been disclosed in Japanese Patent Provisional Publication No. 9-210175 (hereinafter is referred to as JP9-210175). In the infinitely variable transmission (IVT) as disclosed in JP9-210175, generally, input shafts of a continuously variable transmission and a first reduction gear unit, which are arranged parallel to each other, are connected to a unit input shaft (connected to an output shaft of an engine). A counter gear is used as a main part of the first reduction gear unit. A CVT output shaft and a power recirculation mode reduction gear unit are connected in parallel with each other. The output of the CVT and the output of the power recirculation mode reduction gear unit are input into a planetary gearset, and thereafter power is transmitted through a unit output shaft to axle drive shafts. The power recirculation mode reduction gear unit includes an input gear arranged coaxially with the unit input shaft, an output gear in meshed-engagement with the input gear and coaxially supported on the unit output shaft, and an output shaft that is rotatable at a preset reduction ratio of the power recirculation mode reduction gear unit. An output gear of the CVT meshingly engages via the counter gear of the first reduction gear unit with a gear of the CVT output shaft, such that the CVT output shaft rotates at the CVT transmission ratio based on engine speed and load. The CVT output shaft is connected to a sun gear of the planetary gearset. The output shaft of the power recirculation mode reduction gear unit is coupled to a single-planet-pinion carrier of the planetary gearset. A power recirculation mode clutch is interleaved between the output gear of the CVT and the output shaft of the power recirculation mode reduction gear unit. A ring gear of the planetary gearset is coupled to the unit output shaft of the IVT. A direct mode clutch is interleaved between the CVT output shaft and the IVT unit output shaft. In case of an infinitely variable transmission mounted on a front-wheel-drive vehicle, a differential, operatively connected to both the axle driveshafts, is accommodated in a transmission housing. A transmission output gear is attached to one end of the IVT unit output shaft such that the transmission output gear meshingly engages a final gear of the differential so as to continuously transmit power (driving torque) from the differential to both the axle driveshafts at a predetermined overall reduction ratio while permitting speed differentiation therebetween. The IVT is operable in a selected one of at least two operating modes, namely a CVT direct mode and a power recirculation mode. Disengaging the power recirculation mode clutch and engaging the direct mode clutch establish the CVT direct mode. During the CVT direct mode, the output is made according to only the CVT transmission ratio of the continuously variable transmission. On the other hand, engaging the power recirculation mode clutch and disengaging the direct mode clutch establish the power recirculation mode. During the power recirculation mode, depending on both the CVT transmission ratio and the reduction ratio of the power recirculation mode reduction gear unit, a so-called unit transmission ratio of the IVT is able to continuously vary from a negative ratio via a zero IVT output speed point (corresponding to IVT speed ratio=0 or IVT unit transmission ratio=∞) to a positive ratio. In the conventional infinitely variable transmission as disclosed in JP9-210175, at straight-ahead high-speed driving of the vehicle, the IVT is operated in the CVT direct mode. Thus, a fuel consumption rate is directly affected by a transmission efficiency of the CVT. At high load operation, the transmission efficiency of the CVT tends to be less than that of a standard planetary-gear equipped automatic transmission. This leads to the problem of deteriorated total fuel consumption rate of the infinitely variable transmission during high-speed driving. On IVTs, it would be desirable to improve fuel economy by enhancing the transmission efficiency of the IVT during high-speed driving.
Accordingly, it is an object of the invention to provide a torque split infinitely variable transmission, which avoids the aforementioned disadvantages.
In order to accomplish the aforementioned and other objects of the present invention, a torque split infinitely variable transmission comprises a continuously variable transmission connected to a unit input shaft of the infinitely variable transmission, a power recirculation mode reduction gear unit connected to the unit input shaft, a torque split mode reduction gear unit connected to the unit input shaft, a planetary gearset comprising a first member coupled to an output shaft of the continuously variable transmission, a second member coupled to a common output shaft common to the power recirculation mode reduction gear unit and the torque split mode reduction gear unit, and a third member coupled to a unit output shaft of the infinitely variable transmission, a power recirculation mode clutch interleaved between an output member of the power recirculation mode reduction gear unit and the common output shaft, for inputting a torque greater than a torque through the unit input shaft into the continuously variable transmission when the power recirculation mode clutch is engaged, a torque split mode clutch interleaved between an output member of the torque split mode reduction gear unit and the common output shaft, for inputting a torque smaller than the torque through the unit input shaft into the continuously variable transmission when the torque split mode clutch is engaged, a direct mode clutch interleaved between the output shaft of the continuously variable transmission and the unit output shaft of the infinitely variable transmission, for inputting the torque through the unit input shaft into the continuously variable transmission when the direct mode clutch is engaged, and the power recirculation mode clutch, the direct mode clutch, and the torque split mode clutch being selectively engaged and disengaged for establishing either of a power recirculation mode, a direct mode, and a torque split mode. It is preferable that the output shaft of the continuously variable transmission and the power recirculation mode reduction gear unit is connected in parallel with each other, and a torque split mode reduction gear and clutch device, including the torque split mode reduction gear unit and the torque split mode clutch, is arranged parallel to a power recirculation mode reduction gear and clutch device, including the power recirculation mode reduction gear unit and the power recirculation mode clutch. It is more preferable that a ratio R3/R13 of a reduction ratio R3 of the power recirculation mode reduction gear unit to a reduction ratio R13 of the torque split mode reduction gear unit is set to be equal to a ratio VLow/VHigh of a maximum transmission ratio VLow of the continuously variable transmission to a minimum transmission ratio VHigh of the continuously variable transmission. Preferably, the continuously variable transmission may comprise a toroidal continuously variable transmission, which has input and output disks coaxially arranged and opposing each other and a direction of rotation of the output disk is reversed relative to a direction of rotation of the input disk.